Multi-energy computed tomography (CT) is an x-ray imaging modality which produces 3D images of the inside of objects. CT scanners use polychromatic x-ray sources which emit a full rainbow of x-rays with various colours (x-ray energies). In regular CT there is no distinction made between the different energies of x-rays. However x-rays are absorbed differently by different materials in the body, and differently again for x-rays of different energies. Multi-energy CT measures the absorption of x-rays in different energy ranges. Using the differences in x-ray absorption in these energy ranges it is possible to discriminate between (identify) and quantify various materials in an object.
A major role of material analysis algorithms in multi-energy CT is to assign to each voxel in the reconstructed CT image the quantity of one or more materials from a dictionary of possible materials which may be present. This poses a significant problem as many of the materials that one would include in this dictionary have very similar x-ray attenuation properties. When the dictionary has a large number of similar materials then the inversion techniques used for material analysis algorithms are numerically unstable—producing nonsensical results.
Another common problem faced by all material detection and quantification algorithms is the incorrect identification of one material as another. There are three types of material misidentification which are typically encountered. The first type results from numerical instability when decomposing into a basis containing a large number of materials. Large material basis sets are ill-conditioned due to the similarities in attenuation between different materials. The second type results from omitting materials from the decomposition to achieve a more stable inversion. This however will project non-represented materials onto the given basis, describing them as a combination of different materials. The third type occurs when material decomposition is done on poor quality data with significant image artefacts such as excessive noise.
The problems above have made it difficult to identify and quantify large numbers of different materials within a multi-energy CT scan. Known material decomposition methods have achieved the discrimination of four different materials (excluding air) or five (including air) using multi-energy CT. However, it would be advantageous to establish a method that is able to discriminate and identify larger numbers of different materials using a multi-energy CT scanning system.